linux-kernel - Re: [PATCH 3/7] reset: lpc18xx: use devm_reset_controller

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Message-ID: <1462273690.3617.56.camel@pengutronix.de>
Date:	Tue, 03 May 2016 13:08:10 +0200
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:	Joachim Eastwood <manabian@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/7] reset: lpc18xx: use devm_reset_controller_register()

Am Dienstag, den 03.05.2016, 19:25 +0900 schrieb Masahiro Yamada:
> Hi Philipp,
> 
> 2016-05-03 18:05 GMT+09:00 Philipp Zabel <p.zabel@...gutronix.de>:
> > Am Dienstag, den 03.05.2016, 00:52 +0900 schrieb Masahiro Yamada:
> >> 2016-05-02 17:26 GMT+09:00 Philipp Zabel <p.zabel@...gutronix.de>:
> >> > Am Sonntag, den 01.05.2016, 19:36 +0900 schrieb Masahiro Yamada:
> >> >> Use devm_reset_controller_register() for the reset controller
> >> >> registration and remove the unregister call from the .remove callback.
> >> >>
> >> >> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> >> >> ---
> >> >>
> >> >>  drivers/reset/reset-lpc18xx.c | 4 +---
> >> >>  1 file changed, 1 insertion(+), 3 deletions(-)
> >> >>
> >> >> diff --git a/drivers/reset/reset-lpc18xx.c b/drivers/reset/reset-lpc18xx.c
> >> >> index 3b8a4f5..dd4f27e 100644
> >> >> --- a/drivers/reset/reset-lpc18xx.c
> >> >> +++ b/drivers/reset/reset-lpc18xx.c
> >> >> @@ -199,7 +199,7 @@ static int lpc18xx_rgu_probe(struct platform_device *pdev)
> >> >>
> >> >>       platform_set_drvdata(pdev, rc);
> >> >>
> >> >> -     ret = reset_controller_register(&rc->rcdev);
> >> >> +     ret = devm_reset_controller_register(&pdev->dev, &rc->rcdev);
> >> >>       if (ret) {
> >> >>               dev_err(&pdev->dev, "unable to register device\n");
> >> >>               goto dis_clks;
> >> >> @@ -229,8 +229,6 @@ static int lpc18xx_rgu_remove(struct platform_device *pdev)
> >> >>       if (ret)
> >> >>               dev_warn(&pdev->dev, "failed to unregister restart handler\n");
> >> >>
> >> >> -     reset_controller_unregister(&rc->rcdev);
> >> >> -
> >> >>       clk_disable_unprepare(rc->clk_delay);
> >> >>       clk_disable_unprepare(rc->clk_reg);
> >> >>
> >> >
> >> > Hmm, would this patch theoretically allow a window between the calls to
> >> > clk_disable_unprepare(clk_reg) and devm_reset_controller_release() where
> >> > reset_control_get() + reset_control_(de)assert() would access unclocked
> >> > registers?
> >>
> >> This is not clear to me.
> >>
> >> Why reset_control_get() + reset_control_(de)assert() would happen here?
> >
> > I suppose on a non-SMP device, without parallel probing this can't
> > really happen in practice.
> > It still seems weird that suddenly we disable the clocks before
> > unregistering the reset controller instead of afterwards.
> >
> 
> I still do not understand what you mean.
> 
> This patch moves the reset_controller_unregister() call
> after clk_disable_unprepare().

And so the register access is made impossible before the reset
controller device actually vanishes from the publicly visible list.

> But, reset_controller_unregister() is just a manipulation of a liked list.
> It does not trigger any hardware access.
> 
> Am I wrong?

No, you are perfectly right. I don't see how this can be a real problem
unless at the same time another driver could try to request the still
available reset control.

regards
Philipp